# A positively charged rod is held near a small, uncharged metal sphere resting on an insulating stand. The sphere is then touched on the far side by a grounded wire, and the wire is removed before the rod is taken away. Explain why the metal sphere ends up with a negative charge.

> Eduqas GCSE Physics — 7.3 Static electricity – forces and electric fields · Explain · 4 marks

## Mark scheme (4 marks)

1. The positive rod attracts free electrons in the metal sphere towards the near side / repels positive ions away from the near side (induced charge separation / redistribution of electrons)
2. Electrons flow from the ground/Earth into the sphere (through the wire) because the far side of the sphere is positively charged / electron-deficient
3. When the wire is removed, the extra electrons are trapped on the sphere (the sphere now has an excess of electrons)
4. When the rod is taken away, the excess electrons spread over the sphere, leaving it with an overall negative charge

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